Electro-optic modulators are used in fiberoptic communication systems, in particular, commercial long distance, high speed digital communication systems. Electro-optic modulators convert electrical signals into modulated optical signals. The electrical signals represent the data or voice for transmission over the fiberoptic communication system. The modulated optical signal is the data/voice transmission in an optical waveform.
Techniques for modulating an optical signal include amplitude modulation and phase modulation. Amplitude modulation results in the modulated optical signal or light emitted from the modulator being switched from on to off. A Mach-Zehnder interferometer is an example of a modulator performing amplitude modulation. Phase modulation results in the phase of the optical signal being shifted a certain number of degrees. Phase modulated signals require specialized wave detectors to detect the change in phase, while amplitude modulated signals can be detected by photon detectors that detect the power of the received signal in terms of photons.
The typical structure of an electro-optic modulator includes an electro-optic substrate, such as lithium niobate or III-V semi-conductors, an optical waveguide pattern defined within the substrate, and an electrode structure disposed over the substrate carrying the electrical signals to be converted.
As electrical signals propagate through the electrode structure, an electric field is generated across a length of the waveguide, known as the interaction distance. The application of the electric field across the waveguide for the entire length of the interaction distance affects the refractive index of the electro-optic substrate causing an optical signal (i.e. light) propagating through the waveguide to be modulated.
The electrode structure disposed over the substrate provides a strong electrical-optical interaction. The interaction strength is characterized by a "VL product," which is the product of a switching voltage (V.sub..PI.) of the electrical signal multiplied by the interaction distance (L) (i.e. V.times.L). The switching voltage, V.sub..PI., is the voltage swing required at the modulator electrical input to cause the light being emitted from the modulator either to go from "on" to "off" or to shift its phase a certain amount. This interaction strength is generally a constant, typically 55 volts per millimeter (V-mm) at 1550 nanometers (nm) for lithium niobate.
The present limiting factor of electro-optic modulators is the high drive voltage requirement suitable for transmitting at data rates of 10 gigabits per second (Gbit/s) and above. According to the VL product, the required switching voltage can be lowered if the modulator's interaction distance is increased such that the electric field is applied to the optical signal over a maximum length. However, long modulators require the velocities of the electrical and optical signals to be matched in order to apply the electric field to the optical signal for the entire length of the interaction distance. If not, the electric field would not be applied to the optical signal for the entire length of the interaction distance degrading the modulation.
There are a variety of electrode designs in existence today that achieve the necessary velocity match. All of them make some compromise among velocity, electrical/optical overlap (a measure of modulation efficiency), impedance, microwave loss, and manufacturability. Combined with modulator substrate materials, primarily lithium niobate and III-V semi-conductors, state of the art modulator performance is barely adequate for 10 Gbit/s systems and is inadequate at higher speeds, such as 40 Gbit/s.
A known electrode design for velocity matching is the capacitive loading of a transmission line. The effect of this design is to slow down the velocity of the electrical signal relative to its velocity on the unloaded transmission line, and to lower the impedance below that of the unloaded line. This is an advantage in modulator designs where the electrical velocity on the unloaded line is faster than the velocity of light in the waveguides, such as modulator designs based on III-V semiconductor electro-optic substrates.
However, the slowing effect of this electrode design renders this type of electrode useless for modulators where the velocity of the electrical signal is already slower than the optical velocity, such as modulator designs based on lithium niobate electro-optic substrates.